Kwatog & Co » sybase http://kwatog.com tech notes and general nonsense Fri, 04 May 2012 07:34:43 +0000 en hourly 1 http://wordpress.org/?v=3.3.2 Force Table Scan in SQL Server/Sybase http://kwatog.com/blog/sql-server/force-tablescan-in-sqlserversybase/?utm_source=rss&utm_medium=rss&utm_campaign=force-tablescan-in-sqlserversybase http://kwatog.com/blog/sql-server/force-tablescan-in-sqlserversybase/#comments Sat, 07 Aug 2010 08:00:25 +0000 admin http://kwatog.com/?p=376 When extracting whole tables or large recordsets (say, at least 80% of the table), it is better to use Full Table Scan than use index. Full scan is more efficient in this case because it no longer need to search through the index for each record. It doesn’t need to because the whole table is going to be extracted anyways.

However, you need to tell SQL Server that you want to use full table scan. Otherwise, it will use an index depending on the where clause or join criteria you have on your query.

Force Table Scan Syntax
SELECT memberID, fname, lname
FROM members WITH (INDEX(0))
WHERE dept_cd = 100

SELECT dept_name, memberID, fname, lname
FROM members WITH (INDEX(0))
,department WITH (INDEX(0))
WHERE members.dept_cd = department.dept_cd

By the way, some DBAs are not aware of this (force table scan). I’m not a DBA but I worked on a lot of performance optimization projects before and has always been working with our DBAs. Their usual advice is to use the index which is correct in most cases. However, I think some of our queries should have use a force table scan to be more efficient.

]]> http://kwatog.com/blog/sql-server/force-tablescan-in-sqlserversybase/feed/ 0 Oracle NVL Equivalent in MySQL http://kwatog.com/blog/mysql/oracle-nvl-equivalent-in-mysql/?utm_source=rss&utm_medium=rss&utm_campaign=oracle-nvl-equivalent-in-mysql http://kwatog.com/blog/mysql/oracle-nvl-equivalent-in-mysql/#comments Tue, 29 Dec 2009 01:59:44 +0000 admin http://kwatog.com/?p=204 Here’s the MySQL equivalent of Oracle’s NVL and MSSQL/Sybase’s ISNULL functions.

Syntax


IFNULL(expr1,expr2)

Usage

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2. Take note that IFNULL() returns a numeric or string value, depending on the context in which it is used. I haven’t used it for other data types yet but I reckon it would have the same results. That is to return the same data type as that of expr1. Another important thing to remember is that expr1 and expr2 should have the same data type

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